1. Field of the Invention
The present invention relates to partial oxidation or autothermal reforming of fuel, and more specifically, the invention relates to segmented catalyst systems for use in reforming of fuel for use in fuel cells.
2. Background of the Invention
Partial oxidation and autothermal reformers convert hydrocarbon and oxygenated fuels into hydrogen and carbon oxides which can be used in fuel cell applications; particularly fuel cell applications constrained by weight and volume, or which require frequent starts and stops, and have to respond to changes in hydrogen demand. Partial oxidation and autothermal reformers are able to meet these requirements because these reactors operate with a feed that consists of fuel and air in partial oxidation systems and fuel, air and steam in autothermal reformers. Oxygen in the reactant mix allows the fuel oxidation/combustion reaction, which is needed to enable the endothermic steam reforming reaction to occur.
The reforming reactors typically use a noble metal catalyst that supports both the oxidation and reforming reactions, with the oxidation zone followed by the reforming zone. The Reforming zone is where oxygen concentration is extremely low.
Generally, a partial oxidation/combustion reaction first occurs as depicted in Equation 1:CnHm+O2→CO+CO2+H2O.  Equation 1
The reaction in Equation 1 is exothermic and provides heat necessary to drive the reforming portion of any autothermal reformer system, the reforming portion depicted in Equation 2:CnHm+H2O→CO+CO2+H2  Equation 2
Generally, the reforming portion (i.e., Equation 2) of the process occurs at relatively low oxygen concentrations.
Temperature profiles consist of a sharp peak that can reach or exceed 1000° C., at which temperature the catalyst activity diminishes over time. In order to reduce the maximum temperature and thereby extend the life of the catalyst, reactor designs vary the air-to-fuel, allow multiple injections of air, and, in the case of autothermal reformers, steam-to-fuel ratios. While reducing the air in the mixture feeds (i.e., making the fuel mix richer) will minimize peak temperatures, it also leads to a lower average temperature and therefore to lower hydrogen yields.
Other attempts to lower system temperature include siphoning heat from exothermic portions of the reaction, warming air with that heat, then injecting that heated air at multiple injection points. However, this has proved counterproductive, inasmuch as the air reacts with any hydrogen produced instead of being utilized to facilitate oxidation of the carbon in the unconverted hydrocarbon.
A need exists in the art for a system for reforming fuel, and a method for reforming fuel that preserves the life of the catalysts used in such scenarios. The method and system should provide for near complete conversion of fuel, while also ensuring that the catalysts last for at least 5000 start-stop cycles, assuming 10 hours per cycle. The method and system should also be initiated with an energy input (in BTUs) which is no more than 1 to 3 percent, and preferably less than one percent of the total energy produced by the system/method during each cycle.